The principal object of an automotive exhaust muffler is to reduce the noise produced by exhaust gases to an acceptable level. There are several other requirements that compete with the noise reduction functions of an automotive exhaust muffler. In particular, the muffler is confined to a relatively small available space envelope on the underside of the vehicle. It becomes more difficult to attenuate noise in smaller mufflers. Another competing factor is the manufacturing cost. This often is an important factor in the highly competitive and mature exhaust industry. A third factor which competes directly with noise reduction is the effect of the muffler on engine performance. More particularly, mufflers inherently impede the flow of exhaust gases from the engine. This impedence creates a back pressure between the muffler and the engine, with a resulting negative effect upon the power production of the engine. Automotive engineers therefore must make design decisions that will achieve a selected noise level while minimizing back pressure and its effects on engine performance.
Many vehicle owners or operators are primarily concerned with the noise attenuation characteristics of an exhaust muffler. Performance penalties that may be associated with a muffler that achieves an acceptably quiet operation are considered to be less significant. Other vehicle owners or operators are primarily concerned with the performance characteristics of the exhaust muffler, and will tolerate somewhat higher noise levels.
Generally, the mufflers that stress improved engine performance are designed to provide a more direct flow path for exhaust gases in an effort to minimize back pressure. A typical high performance muffler includes opposed inlet and outlet ends with corresponding inlet and outlet tubes or nipples which extend into the body of the muffler. The typical prior art high performance muffler includes three internal chambers spaced along the length of the muffler. For purposes of this disclosure, the chambers nearest the inlet and outlet ends of the muffler will merely be referred to as the inlet end chamber and the outlet end chamber. These chambers often are referred to as flow chambers. The central chamber, as explained below, often is a low frequency tuning chamber. The inlet tube will extend through the inlet end chamber and the central chamber and will enter into the outlet end chamber. The inlet tube often will be of solid wall construction through the inlet end chamber but will be perforated in the central chamber of the muffler. The inlet tube typically is offset with respect to the longitudinal axis of the muffler. In a similar manner, the outlet tube will extend through the outlet end chamber and the central chamber of the muffler and will open into the inlet end chamber thereof. The outlet tube generally will be of solid wall construction in the outlet end chamber, but may be perforated in the central chamber of the muffler. The outlet tube typically is centrally located relative to the longitudinal axis of the muffler. A transfer tube is provided to provide communication between the inlet end chamber and the outlet end chambers, or the chambers disposed on opposite ends of the muffler. This transfer tube may also be of perforated construction as it extends through the central chamber of the muffler.
The central chamber of the prior art muffler often is packed with a sound absorbing material around the three perforated tubes extending therethrough. A typical sound absorbing material would be fiberglass. Absorbing packings such as fiberglass can be somewhat effective in reducing noise. However, the very hot exhaust gases flowing through the muffler at high speeds tends to break up the absorbing materials. In most applications the absorbant will literally be blown out of the exhaust system well within the anticipated structural life of the muffler. Thus, although the above described prior art high performance mufflers may provide acceptable noise levels when they are first installed, they will become increasingly noisy over time.
Other prior art high performance mufflers have attempted to reduce noise by varying the construction of the opposed heads or end caps of the muffler. The end caps define the respective longitudinal ends of the muffler through which the inlet and outlet pipes extend. In one prior art muffler, the opposed end caps defined a continuous arcuate structure disposed convexly outward. This construction was found to reduce the vibrations of the end caps and to reduce the noise associated with such vibrations. However, this construction often is considered to present certain manufacturing difficulties. More particularly, the convex outward end caps must be mated with a tubular sidewall. This mating must be structurally secure and typically is entirely mechanical, with no welding. It has been found that the arcuate end cap and the tubular sidewall can be difficult to connect. Similar difficulties may be encountered in securely connecting the tubular inlet or outlet pipe with the arcuate end cap.
Other prior art high performance mufflers have attempted to attenuate noise by mounting separate dish-shaped members to the inner surface of each opposed end cap. The dish-shaped members are intended to be of generally parabolic shape and are disposed such that the respective concave surfaces are inwardly facing. Each dish-shaped member further includes a generally annular support wall which extends from the rim of the dish toward the associated end cap of the muffler. These dish-shaped deflectors are purported to reflect the sound waves produced by the exhaust gases and to create an oppositely directed array of sound waves to cancel the principal array of sound waves. The ability of these dish-shaped deflectors to reliably cancel a significant portion of the oncoming sound waves is considered to be somewhat questionable. More particularly, the sound waves produced by the exhaust gases are not of a single frequency and wavelength but cover a fairly broad spectrum. Consequently, the dish-shaped deflectors can only be effective in cancelling a very narrow range of the sound produced by the exhaust gases. Furthermore, the dish-shaped deflectors extend outwardly from the associated muffler end caps and create an irregular internal surface for the muffler. This surface is believed to create turbulence with a somewhat negative effect on back pressure and engine performance. Furthermore, this additional turbulence is not believed to be engineered in a manner that will create a noise attenuation effect. In addition to the above described functional deficiencies of the dish-shaped deflectors, it is apparent that these structures add significantly to the manufacturing costs and time.
Certain prior art mufflers that are engineered primarily for their noise attenuation attributes rather than for high performance are known to include a plurality of internal chambers of varying dimensions. The varying size chambers each have their own noise attenuation characteristics. More particularly, the small chambers tend to be effective in attenuating high frequency noises, while the larger chambers tend to be effective in attenuating low frequency noises. The prior art high performance mufflers generally have not been provided with these various chambers. Rather, as explained above, the prior art high performance mufflers have primarily been engineered to yield a fairly direct air flow path with minimum back pressure.
In view of the above, it is an object of the subject invention to provide an improved high performance automotive muffler.
It is another object of the subject invention to provide a high performance muffler with an enhanced ability to attenuate sound produced by exhaust gases passing therethrough.
It is an additional object of the subject invention to provide an automotive muffler that will substantially reduce back pressure produced by the muffler.
Another object of the subject invention is to provide a high performance muffler with an enhanced ability to attenuate high frequency sounds.
Still another object of the subject invention is to provide a high performance muffler that can be manufactured efficiently and inexpensively.